In a cellular communication system, when service data is transmitted between two user equipment (UE), for example, the service data from a user equipment 1 (UE1) to a user equipment 2 (UE2) is first transmitted, via an air interface, to a base station (e.g., a Node B or an evolved Node B (eNB) in the third generation mobile communication) of a cell where the UE1 is located. The base station transmits the service data to a base station of a cell where the UE2 is located through a core network, and then the base station of the cell where the UE2 is located transmits the service data to the UE2 via the air interface. The service data is transmitted from the UE2 to the UE1 using the similar processing procedure. As shown in FIG. 1a, when the UE1 and the UE2 are located in the same cellular cell, although the two UEs are covered by the cell of the same base station, the data still needs to be transferred via the core network upon transmission, and two radio spectrum resources will still be consumed in a single data transmission process.
Thus it can be seen that the cellular communication method described above is not optimal if the UE 1 and the UE 2 are located in the same cell and closer to each other. However, actually, as mobile communication services become diversified, for example, popularization of applications such as social networks, e-payment and the like in wireless communication systems causes the demand for service transmission between users at close range to grow increasingly. Thus, a device-to-device (D2D) communication mode is received extensive attention. As shown in FIG. 1b, the D2D communication mode, which is also called proximity service (ProSe), means that the service data is transmitted directly from a source UE to a target UE via the air interface without forwarding by the base station or core network. For the users communicating with each other at close range, the D2D not only saves radio spectrum resources but also reduces the data transmission pressure of the core network.
In cellular communications, when two UEs are communicating with each other, one UE is generally unaware of the location of the other UE, thus a communication link with the other UE is required to be established via a network side device (e.g., a base station or core network device). For the device-to-device communication, the precondition for the establishment of the communication link is the mutual discovery between the UEs. Because of the half-duplex characteristic of the D2D communication, the UEs are incapable of receiving device discovery signals when sending the device discovery signals or incapable of sending device discovery signals when receiving the device discovery signal. As a result, when the UEs send the device discovery signals at the same time, they cannot discover each other, which limits the application of the D2D communication.